Apple Watch won’t stay connected to wifi
Instantly disconnects from wifi, phone connects with no issues
Apple Watch SE
Did you already try to force restart the watch by pressing the Digital Crown and the Side button together for at least 10 sec, until the Apple logo comes back again?
If this does not work, erasing the watch and setting it up again as a new device would be the next step:
Unpair and erase your Apple Watch - Apple Support
If Wi-Fi works after that, restore from the latest backup.
In case the issue comes back, the backup data is corrupted and can’t be used anymore after erasing and pairing it again.
Also make sure that your Wi-Fi supports WLAN 4 (802.11n)